Helping You Learn and Leverage Computer Networking

Getting to Know your Neighbor

A Puzzle Without a Picture

Ever try to put a puzzle together without a picture to go by? Ever try to find your way around a new city without referencing a map? Having a simple guide or reference helps tremendously when trying to get around. It is critical when trying to solve a problem, especially when the pressure is on to get it solved. When troubleshooting, if you have no reference point, you have to create one before you can move forward. So, in the networking world, where can you get a map? What do you use as a reference picture when you set about solving a networking puzzle?

When troubleshooting, if you have no reference point, you have to create one before you can move forward.

Going Door to Door to Create a Map

If you don’t have documentation or your documentation is out of date, troubleshooting a network problem can be a challenge. We all know this, but we get busy and documentation can easily fall by the wayside, or we inherit an undocumented network and don’t know where to begin. So, what now? How can I easily create a physical map of my network? You can use a feature called LLDP or CDP to figure out the physical layout of your network. If you are using managed switches and routers they probably have a feature on them called CDP or LLDP. CDP (aka, Cisco Discovery Protocol) and LLDP (Link Layer Discovery Protocol) are a simple and very useful feature. They difference between the is that CDP is proprietary to Cisco while LLDP is an open standard. Both are basically a simple way to figure out what network devices are plugged into each other.

You can use a feature called LLDP or CDP to figure out the physical layout of your network.

How LLDP and CDP Work

Managed network devices will send LLDP or CDP packets out roughly once a minute on all network ports. These packets include the following key information.

Name – The name of the device sending the packet

IP – The IP of the device sending the packet

Port – The port info of the sending device that the packet was sent on

VLAN – The native VLAN for the port that the packets was sent on

The other network devices that are directly connected to the device that sent the LLDP/CDP packets will note what port it received the packet on and the information in the packet. These packets are not forwarded on to any other device. The result is that every device that supports LLDP/CDP creates a list of all the network devices it is directly connected to and on what port it is connected on.

Every device that supports LLDP/CDP creates a list of all the network devices it is directly connected to.

The Meaning of it All

When I first learned about this feature I thought, “That’s nice, but I doubt I’ll use it that much.”. I was totally wrong. This is a feature that you can use all the time. It helps identify loops in your network (you see the same neighbor switch twice, or you see the switch you are on as a neighbor to itself), it helps locate where users connect into the network (using a device that understands LLDP/CDP like a Fluke LinkRunner), it helps you see if a computer/printer/etc. is in the right VLAN for the IP that it is using, and it helps you trace cabling all over the building from the comfort of your desk.

LLDP and CDP enable you to trace network cables from the comfort of your desk.

So have you ever used LLDP or CDP in your environment? In what ways has this simple feature helped you?